Role of Ceramide Synthase in Oxidant Injury to Renal Tubular Epithelial Cells

. Ceramide has been implicated to play an important role in the cell signaling pathway involved in apoptosis. Most studies that have used the apoptotic model of cellular injury have suggested that enhanced ceramide generation is the result of the breakdown of sphingomyelin by sphingomyelinases. However, the role of ceramide synthase in enhanced ceramide generation in response to oxidant stress has not been previously examined in any tissue. Hydrogen peroxide (H 2 O 2 ) (1 mM) resulted in a rapid increase in ceramide generation (as measured by in vitro diacylglycerol kinase assay) in LLC-PK1 cells. The intracellular ceramide level was significantly increased at 5 min after exposure of cells to H 2 O 2 and thereafter continuously increased up to 60 min. H 2 O 2 also resulted in a rapid increase (within 5 min) in ceramide synthase activity (as measured by incorporation of [ 14 C] from the labeled palmytoyl—CoA into dihydroceramide) in microsomes. In contrast, the exposure of cells to H 2 O 2 did not result in any significant change in sphingomyelin content or acid or neutral sphingomyelinase activity. An increase in ceramide production induced by H 2 O 2 preceded any evidence of DNA damage and cell death. The specific inhibitor of ceramide synthase, fumonisin B1 (50 μM), was able to suppress H 2 O 2 -induced ceramide generation and provided a marked protection against H 2 O 2 -induced DNA strand breaks, DNA fragmentation, and cell death. Taken together, these data provide the first evidence that H 2 O 2 is a regulator of ceramide synthase rather than sphingomyelinases and that ceramide synthase—dependent ceramide generation plays a key role in DNA damage and cell death in oxidant stress to renal tubular epithelial cells.